Background: Radiation segmentectomy (RS) is the selective trans-arterial delivery of an ablative dose of radiation to tumor-bearing hepatic segments, with the goal of complete tumor necrosis comparable to resection or thermal ablation. The technique was first described in patients with unresectable hepatocellular carcinoma (HCC). RS has since been applied to intrahepatic cholangiocarcinoma (iCCA) and to liver metastases of diverse origin, including colorectal, neuroendocrine, breast and sarcoma primaries. Purpose: To synthesize the original-data evidence base for RS across tumor types, to demonstrate the impact of tumor vascularity on tumor-specific dosimetry methods, and to define where the evidence supports standardization of methodology. Findings: Evidence is strongest for HCC, where pathologic-explant studies tie ablative dose thresholds to complete pathologic necrosis and prospective and large-cohort data demonstrate high response rates and extended patient survival. For colorectal liver metastases, several studies have associated tumor absorbed dose with response and local control. A single study additionally suggests that the dose delivered to the peritumoral margin is associated with local tumor control, an interesting hypothesis that requires further prospective validation. Data in iCCA and neuroendocrine metastases support feasibility and durable responses. The first international dosimetry consensus for non-HCC indications appeared in 2025. Heterogeneity in dosimetry models, dose thresholds, patient selection, and outcome reporting limits cross-study pooling. Conclusions: RS serves as a potentially curative locoregional therapy for selected patients with early-stage HCC and represents a promising treatment option for non-HCC tumors, especially for those not amenable to resection or thermal ablation. Histology-specific dosimetry, prospective validation, especially in non-HCC disease, and consensus reporting standards are necessary.